The invention relates generally to charge transfer devices and, more particularly, to a time-independent CCD circuit for multiplying and adding charge.
The basic operation of charge-coupled devices has been explained in detail in the technical and patent literature, but a brief summary of the operation of such devices may facilitate an understanding of the present invention. While the structure of a charge-coupled device will be given in terms of specific semiconductor material types, it will be understood that in general where P-type material is specified, N-type material may be substituted and vice versa.
A typical charge-coupled device may consist of a P-type silicon substrate (in which electrons are normally the minority signal carriers) with a silicon dioxide insulating layer superimposed on its surface. An arrangement of conducting electrodes is deposited on the surface of the insulating layer.
When clock voltages are applied to predetermined groupings of the electrodes, some of the electrons in the vicinity of each electrode, assuming that electrons are initially present (as a result, for example, of injection into the device), will form a discrete packet of charge and move one charge-coupled element, or unit cell, in a predetermined direction for each full clock cycle. The packets of charge move in the predetermined direction as a result of the continuous lateral displacement of the local potential well in which they find themselves. Charge-coupling is thus the collective transfer of all the mobile electric charge stored within a semiconductor storage region to a similar, adjacent storage region by the external manipulation of clock voltages.
The quantity of charge capable of being stored in the mobile packet can vary widely, depending on the applied voltages and on the capacitance of the storage regions. The amount of electric charge in each packet can represent information. Charge-coupled devices have utility in photosensor arrays, delay lines, shift registers, buffer memories, sequential-access memories, fast-access scratchpad memories, refresh memories, and other information storage and transfer mechanisms.
Various types of CCD charge amplification and distribution circuits are known in the prior art. Several of these are described in Charge Transfer Devices, Sequin and Tompsett, Academic Press, Inc., New York, 1975. For example, at page 56 et seq. this publication describes a floating gate distributed amplifier in which charge is sampled at particular points in a first CCD device, amplified, and input into corresponding points in a second, larger CCD device, so that the signal initially present in the first CCD device is amplified coherently in the second CCD device. U.S. Pat. No. 3,806,772 describes a similar floating gate CCD distributed amplifier. At page 216 et seq. the aforementioned publication further discloses the use of CCD's for signal processing in the form of transversal filters having either fixed or variable tap weights. These devices utilize a circuit in which a first CCD line is non-destructively tapped at several locations, each tap being multiplied by a different weighting coefficient. The weighted taps are then combined in a differential summing amplifier or other suitable accumulator circuit. The publication also discloses several two-dimensional transfer arrays (p. 261 et seq.) in which charge may be transferred in up to four possible directions from a given charge storage area of the array. Circuits are also described therein for binary adders and multipliers (p. 270 et seq.) by means of which charge is multiplied and accumulated in specific ways.
In the known prior art CCD circuits for amplifying and accumulating charge, the amount of charge which is amplified by the individual charge amplifiers is dependent upon the length of time the input gates of the amplifiers are turned on and upon the threshold voltages of the electrodes and the semiconductor substrate. The amplified charge is thus rather imprecise and is influenced both by variations in operational frequency as well as by variations in clock voltages.